Soil stack air admittance valve issue?

I live in a second floor apartment and have been struggling to find the cause of a sewer smell in the bathroom since I moved in. The waste pipe doesn't have an external vent but has a AAV on the top of the stack on the 4th floor. When the AAV is removed from the stack the issue is resolved in my apartment. All of the drains have been jetvac'd recently and I've been assured they're clear. No one else connected to the soil stack is complaining of an issue. Does this mean that there is potentially a gap in my own soil pipe (there is no leakage or water collecting anywhere) for the smell to vent when the valve is replaced? My soil pipe is routed behind tiles etc so before removing them to investigate further (big job) I'd like to know where similar issues have been pinpointed before...

Modern push button toilets have a built in overflow that discharges into the pan. You can see the water flowing when the inlet valve isn't closing properly. Older ones have a separate pipe which usually discharges outside (the small 1" pipe you see sticking straight out of the wall). Some overflow pipes discharged into a small tundish trap (visual warning again) before being connected to the internal waste.
What I was referring to was where the overflow had been connected to the internal waste without any tundish or trap so that smells could escape through the pipe, into the cistern and then into the room.
I hope this explains it more clearly.

Hi
I have found that if you remove the aav from the soil stack, dismantle it, and clean the plastic diaphragm type part,

The aav works on the principle of negative pressure in the stack will lift the diaphragm and allow air in the stack.
What can happen, after months or years of use, the steam and vapour from baths etc. can make th diaphragm sticky, and when it lifts to let air in, it sticks and lets sewer gas out!

@Willy - I've checked and the overflow goes back into the bowl so I don't think that should be an issue.

@Peter - The issue isn't that sewer gas is escaping from the vent, it's coming out of somewhere else. When the stack is vented at the top it removes the issue for me, when the AAV is returned to the top of the stack the issue returns almost immediately within my apartment.

It's just guesswork but after checking the soil connectors for leaks checking the traps( and importantly overflows) if the aav is not working this could set up positive pressure in certain circuits. And it may be enough to cause a stink.
For instance I replaced my open vent with an external aav. Now in summer I notice a smell around the adjacent grid! This sounds very similar.

That's my issue at the moment, the soil pipe connectors cannot be checked without starting to take apart my bathroom. A loose connection would surely be causing a leak and I've not been able to find any signs of any leaks whatsoever. It doesn't seem to make any sense as to why i'm able to smell the sewer and yet have no leaks or empty p traps...

Hi, any updates OP? Did you manage to resolve the issue? I have a similar issue in my 3rd floor apartment and for the life of me can not work out why the soil pipe always smells of (sewer gas, mouldy musty and mildew type of smell) behind the plasterboard. Smell is always present and it gets worse every weekend when people above me are at home. I had plumbers in who cut a chunk out of the plasterboard and they couldn’t find any water leaks etc and advised me all joints to the main soil pipe looks fine. They have advised me next step would be to put a camera down from 4th floor to check for cracks etc. Is this necessary or can I look for other clues? Bathroom and kitchen are fine without any smell whatsoever.

I live in a 4 storey building and nobody else is effected by this. I have been residing here for 18 months and the smell started intermittently but now the smell is here everyday and spread around the flat very quickly from the plumbing room.

Hi
I have found that if you remove the aav from the soil stack, dismantle it, and clean the plastic diaphragm type part,

The aav works on the principle of negative pressure in the stack will lift the diaphragm and allow air in the stack.
What can happen, after months or years of use, the steam and vapour from baths etc. can make th diaphragm sticky, and when it lifts to let air in, it sticks and lets sewer gas out!

I' currently on holiday so have put investigations on hold for a few weeks. The smoke test wasn't completed and the management company keep putting it off, I think because of how potentially invasive it will be to other apartments. I'l keep pushing for it though. I have since been visited by another plumber who doesn't think the pressure is correct within the plumbing system. When they fill the sink up, flush the toilet and then let the sink drain air bubbles start to rise through the sink plug. They are going to put an additional AAV on top of the stack to allow more air in. The plumber doesn't think I have any gaps in pipes but rather a build up of pressure which is holding too much sewer gas and it eventually finds its way into my bathroom via traps etc. I'm not fully convinced.

I had a camera as I said before which didn't show anything up. How quicky are your sinks/toilets/baths draining? If they are draining slowly it could point to a similar issue which is being compounded by upstairs using the bathroom and waste getting trapped due the negative pressure?

Let me know how you get on. If you can try and remove the AAV for an afternoon to see if that resolves your issue? I would check with the management company first though - if that is how your building is managed.

I' currently on holiday so have put investigations on hold for a few weeks. The smoke test wasn't completed and the management company keep putting it off, I think because of how potentially invasive it will be to other apartments. I'l keep pushing for it though. I have since been visited by another plumber who doesn't think the pressure is correct within the plumbing system. When they fill the sink up, flush the toilet and then let the sink drain air bubbles start to rise through the sink plug. They are going to put an additional AAV on top of the stack to allow more air in. The plumber doesn't think I have any gaps in pipes but rather a build up of pressure which is holding too much sewer gas and it eventually finds its way into my bathroom via traps etc. I'm not fully convinced.

I had a camera as I said before which didn't show anything up. How quicky are your sinks/toilets/baths draining? If they are draining slowly it could point to a similar issue which is being compounded by upstairs using the bathroom and waste getting trapped due the negative pressure?

Let me know how you get on. If you can try and remove the AAV for an afternoon to see if that resolves your issue? I would check with the management company first though - if that is how your building is managed.

Regards

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Hey Pal,
I’m having nightmares trying to sort this out. My sinks/toilets/baths draining & traps all good with no issues etc. The intermittent rotten egg smell which comes from the plumbing room (where the main soil stack is and all waste pipe join) it comes on randomly anytime of the day or night and can last for anything between 2 hours to 12 hours. Mostly comes out weekends. I’m pretty sure there is a air leak behind the wall or something.

I had camera work done from above 4th floor and they couldn’t find any leaks or cracks etc. Now the next thing they are planning to do is check behind my toilet etc.